For many high-temperature and transient applications, thermocouples are suitable temperature sensors due to their simple design, fast response times, and ability to accommodate unique installation geometries. However, conventional thermocouples cannot be adequately characterized in-situ to determine the thermocouple's time constant.
The time constant is a valuable parameter in correcting for lag associated with the thermal response of a sensor when subjected to highly transient processes or environments. A sensor's time constant provides a quantitative metric of how fast or slow the sensor responds to a change in ambient conditions. However, conventional thermocouples cannot be adequately tested in-situ and analyzed to obtain the thermocouple's time constant.
The conventional method for determining temperature sensor time constant is referred to as the plunge test. Typically, the time constants of resistance temperature detectors (RTD) (i.e., temperature sensors) have been characterized by a single variable called the plunge time constant, which refers to an amount of time required for the sensor output to achieve 63.2% of its final value after a step change in temperature is imposed on the sensor's surface. A step change in temperature is imposed in a testing environment by suddenly drawing the sensor from one medium at an initial temperature to another medium, usually water flowing at 1 ms-1, at a different temperature. However, the plunge test method is deficient in that this method does not account for an influence of process conditions and/or installation of the sensor on the time constant of the sensor.
In order to address the problems with the plunge test, the Loop Current Step Response (LCSR) test method was developed. The LCSR in-situ test method is based on heating a temperature sensor internally by applying a step change in current applied to leads of the sensor. The current heats the sensing element of the sensor and the sensor's temperature rises as a function of the magnitude of the supplied current and the rate of heat transfer between the sensor and its surroundings (e.g., host material). The resulting temperature transient is then analyzed to provide a time constant. As a result, the LCSR test method provides in-situ time constants of the sensors, which are more accurate and precise than time constants determined by the plunge test before sensor installation.
Therefore, what is desired is a thermocouple designed to be analyzed in-situ and a system and method incorporating such thermocouple which monitors degradation of a material in contact with the thermocouple based on sensor interface conditions.